feat(llama2): add template for chat messages (#782)

Co-authored-by: Aman Karmani <aman@tmm1.net>

Lays some of the groundwork for LLAMA2 compatibility as well as other future models with complex prompting schemes.

Started small refactoring in pkg/model/loader.go regarding template loading. Currently still a part of ModelLoader, but should be easy to add template loading for situations other than overall prompt templates and the new chat-specific per-message templates
Adds support for new chat-endpoint-specific, per-message templates as an alternative to the existing Role: XYZ sprintf method.
Includes a temporary prompt template as an example, since I have a few questions before we merge in the model-gallery side changes (see )
Minor debug logging changes.